The embodiments discussed herein relate to a rotor for an electrical machine, in particular a synchronous machine,                wherein the rotor has a rotor shaft defining a rotor axis,        wherein the rotor has a winding support, which is arranged fixed against rotation on the rotor shaft, with the result that it surrounds the rotor shaft in a central section, in relation to the rotor axis,        wherein the winding support supports a superconducting rotor winding,        wherein the rotor has a sleeve, which surrounds the winding support and the rotor winding at least radially, in relation to the rotor axis.        
The embodiments discussed herein furthermore relate to an electrical machine, in particular a synchronous machine,                wherein the electrical machine has a machine housing,        wherein such a rotor is mounted rotatably in the machine housing,        wherein a stator, which surrounds the rotor radially, in relation to the rotor axis, is arranged fixed against rotation in the machine housing,        wherein the stator supports a stator winding, which can be connected to a voltage supply.        
Such a rotor and such an electrical machine are known, for example, from DE 103 00 269 A1.
The sleeve is required, both in the prior art and in the embodiments, in order to be able to cool the superconducting rotor winding to a sufficient extent and to shield it thermally with respect to the surrounding environment. The sleeve needs to have high mechanical strength in order to be able to absorb the mechanical loads occurring during operation. Furthermore, the sleeve needs to have high electrical conductivity in order to absorb unavoidable electromagnetic harmonics.
In DE 103 00 269 A1, the sleeve is formed with two layers. It consists, at least in the region in which it surrounds the winding support and the superconducting rotor winding radially, a radially inner layer consisting of steel and a radially outer layer consisting of copper.
The design of the rotor is complex in DE 103 00 269 A1. Either a steel tube and a copper tube with a very precise fit need to be manufactured or two plates of copper and steel are connected to one another and bent back to form a tube, with the abutting edge being welded. Both procedures are complex and expensive. Furthermore, the welded seam represents a weak point in the construction.
U.S. Pat. No. 4,465,106 A has likewise disclosed a rotor for an electrical machine in which the winding support is surrounded by a multilayered sleeve. The multilayered sleeve has a central layer which consists of a copper/chromium or a copper/zirconium alloy. It is surrounded radially on both sides by iron-containing layers.
U.S. Pat. No. 3,194,655 A has disclosed a copper/chromium/zirconium alloy which has a relatively high material strength. The alloy consists of from 0.6 to 0.9 percent by weight of chromium, from 0.4 to 0.6 percent by weight of zirconium and the remainder copper.